DESCRIPTION In this application, the hypothesis to be tested is that H202 derived from inhaled ozone primes lung mast cells to antigen. To test this, two specific aims are proposed: 1) To determine the effects of administered H202 on allergic responsiveness in ovalbumin sensitized rats. This will be examined by determining the extent to which H202 potentiates ovalbumin induced contraction in airways isolated from sensitized rats. 2) To examine the role of H202 in ozone-induced changes in allergic responsiveness in ovalbumin sensitized rats. This will be tested by determining the ability of catalase to inhibit the effect of ozone on ovalbumin-induced responses in airways isolated from sensitized rats. To rule out airway hyperresponsiveness as a mechanism of H202-induced effect on allergic responses, effect of H202 on 5-hydroxytryptamine (5-HT) induced contraction will be measured. To determine the extent to which mast cell degranulation is induced by H202, levels of 5-HT will be measured following ozone exposure, catalase will be administered to rats to determine the role of H202 in the ozone-induced effects on allergic responses in ovalbumin sensitized rats. It is hoped that these studies will establish whether H202 as a ubiquitious extracellular oxidant and will effect mast cell function.